1. Field of the Invention
The present invention relates to an imaging apparatus and an imaging method.
2. Description of the Related Art
When images are taken, there can be cases where the dynamic range of the luminance of the imaging scene is beyond the input dynamic range of the imaging apparatus, such as a digital still camera and a digital video camera. In such a case, portions of the imaging scene having a luminance out of the input dynamic range are photographed to be blacked out (black fall) or whited out (white raise) in the taken picture, losing their information about shapes and colors.
In order to solve this problem, various imaging apparatus having a wide input dynamic range are proposed, one of which is an imaging apparatus which performs photographing a plurality of times under different light exposure conditions, that is, so-called multiple exposures, to obtain image signals of a plurality of images of different exposure amounts, and thereafter synthesizes the image signals to generate a “synthesized image signal” having a wide input dynamic range.
Such an apparatus needs to calculate the light exposure condition of each photographing operation so as to minimize the extent of the occurrence of the phenomena of black fall and white raise in the synthesized image signal when the phenomena of black fall and white raise occur in the image signals before synthesizing. In this case, an effective method is to calculate a plurality of light exposure conditions for photographing a plurality of times, that is, a light exposure condition group, using a histogram of the luminance distribution of an imaging scene obtained in advance by any technique.
FIG. 6 is a view describing the technique for calculating a light exposure condition group using a histogram, described in Japanese Patent Application Laid-Open No. 2002-135648 (Patent Document 1). A light exposure condition group is calculated that is optimal for the peak portions of frequency in a histogram calculated from the luminance distribution of an imaging scene.
FIG. 7 is the conventional technique for synthesizing raw image signals of two images of different exposure time periods, described in “Wide dynamic range image synthesis processing technique”, National Technical Report Vol. 43, No. 4, August 1997, pp 455-460 (Non-Patent Document 1). A raw image signal (hereinafter referred to simply as an image signal) of a raw image of “a long period of accumulation time” (hereinafter referred to as a light exposure of a long time period) and an image signal of “a short period of accumulation time” (hereinafter referred to as a light exposure of a short time period), obtained from the CCD, are multiplied by a gain comprising the ratio of time periods of accumulation, and thereafter are added together to produce a weighted sum. Here, as for the technique of weighting, the weighting is determined so as to provide a linear change from an image signal of a light exposure of a long time period to an image signal of a light exposure of a short time period in the range from 80% to 100% of the output dynamic range of the image signal of the image of a light exposure of a long time period.
In other words, with respect to the operation of synthesizing using a reference value as the boundary, in the above example, the value of “80% of the output dynamic range of the image signal of the image of a light exposure of a long time period” is used as the reference value, and synthesizing is performed using that value as the boundary. In addition, with respect to the actual operation of synthesizing, the operation of “multiplying long and short exposure time signals by a gain comprising the ratio of time periods of accumulation and thereafter adding together these products to product a weighted sum” is performed.
In the technique of Patent Document 1, although an optimum light exposure condition group can be obtained for an imaging scene of a condition as shown in FIG. 6, there are scenes for which an optimum light exposure condition group cannot be calculated. The problem is described below.
FIG. 8 is the luminance distribution of an imaging scene including three objects in the scene having different brightnesses, converted to a histogram.
The technique shown in Patent Document 1 detects “peak portions”. Therefore, in this case, three ranges represented by 1201, 1202, 1203 are detected and an attempt is made to calculate light exposures which are optimum for these ranges. Only two exposures are permitted for the three ranges, and it is difficult to obtain an optimum light exposure condition group.
The object of the present invention is to determine an appropriate light exposure condition to thereby to generate an appropriate synthesized image signal for an imaging scene for which an appropriate light exposure condition cannot be obtained by conventional techniques, as illustrated above.
The imaging apparatus of the present invention includes: a light exposure condition determining unit for determining a plurality of light exposure conditions based on a histogram of the luminance distribution of an imaging scene; an imaging unit for imaging, at plural times, the imaging scene under the plurality of determined light exposure conditions, and generating a first raw image signal and a second raw image signal; and an image synthesizing unit for synthesizing the first raw image signal and the second raw image signal using, as a boundary, a reference value in the first raw image signal to generate a synthesized image signal, wherein the light exposure condition determining unit determines at least one of the plurality of light exposure conditions so that the reference value in the first raw image signal corresponds to the luminance value at the bottom of a ravine of the histogram of the luminance distribution of the imaging scene.
The imaging method of the present invention includes: a light exposure condition determining step for determining a plurality of light exposure conditions based on a histogram of the luminance distribution of an imaging scene; an imaging step for imaging, at plural times, the imaging scene under the plurality of determined light exposure conditions, and generating a first raw image signal and a second raw image signal; and an image synthesizing step for synthesizing the first raw image signal and the second raw image signal using, as a boundary, a reference value in the first raw image signal to generate a synthesized image signal, wherein the light exposure condition determining step determines at least one of the plurality of light exposure conditions so that the reference value in the first raw image signal corresponds to the luminance value at the bottom of a ravine of the histogram of the luminance distribution of the imaging scene.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.